This section provides background information related to the present disclosure which is not necessarily prior art.
Laser welding is commonly used to weld plastic parts together. Two known techniques are trace laser welding and simultaneous laser welding. In trace laser welding, a spot laser tracks a weld path by movement of the laser device and/or laser beam, work piece, or a combination thereof. The weld path is where the two parts are welded together at a weld interface and may for example be a line extending adjacent a periphery of the parts. In simultaneous laser welding, the full weld path or area (referred to herein as the weld path) is simultaneously exposed to laser light, such as through a coordinated alignment of a plurality of laser light sources, such as laser diodes or through an optical waveguide. In an example of simultaneous laser welding, the laser light is transmitted from a plurality of laser light sources, such as laser diodes, to the parts being welded through one or more optical waveguides which conform to the contours of the parts' surfaces being joined along the weld path.
In both trace laser welding systems and simultaneous laser welding systems, the lasers are often controlled using closed loop feedback. A closed loop feedback control system for a through transmissive infrared (“TTIr”) laser welding system is disclosed in U.S. Pat. No. 7,343,218 for Automatic Part Feedback Compensation for Laser Plastics Welding that is commonly owned with present application. The entire disclosure of U.S. Pat. No. 7,343,218 is incorporated herein by reference. FIGS. 2 and 4 of U.S. Pat. No. 7,343,218 are included herein as FIGS. 1 and 2. Referring FIG. 1, a feedback control system 10 is employed to provide feedback information in a TTIr laser welding system 11 to monitor the intensity of laser light downstream from an infrared laser source 14. Feedback control system 10 comprises an optical sensor 16 that senses infrared radiation that is positioned downstream from laser source 14, yet upstream from plastic parts 22, 24 that are received in laser welding system 11 in abutting relation to each other. Plastic part 22 is illustratively a transmissive plastic part 22 and a plastic part 24 is illustratively an absorptive plastic part that is at least partially absorptive to laser light emitted laser source 14 and a controller 17. It should be understood that plastic part 24 can be made of a material that is at least partially absorptive the laser light, the absorptivity provided by an absorptive weld additive placed at a weld interface between plastic parts 22, 24, or both. In an example, laser source 14 is an infrared laser diode and optical sensor 16 is a photodiode. Controller 17 is coupled to optical sensor 16 and receives from optical sensor 16 on a real-time basis infrared radiation intensity senses by optical sensor 16 due to the intensity of the laser light from laser source 14. Controller 17 is coupled to laser source 14 and controls an output intensity of laser source 14.
In some cases, optical sensor 16 can be positioned upstream of a fiber optic member 18 and/or a waveguide 20, or can be positioned downstream of one or more of fiber optic members 18 and waveguide 20. In other words, optical sensor 16 can be positioned at any position between laser source 14 and part 22.
In some laser welding systems, optical sensor 16 is situated to sense intensity of infrared radiation in a laser chamber in which laser source 14 is situated. As shown in FIG. 2, laser source 14 is situated in a laser chamber 26. Optical sensor 16 senses the intensity of infrared radiation in laser chamber 26 to sense the intensity of laser light that is provided by laser source 14. In the example of FIG. 2 laser source 14 is a laser diode, optical sensor 16 is a photodiode and laser chamber 26 is a laser diode chamber.
A difficulty in sensing in a laser chamber the intensity of laser light that is provided by laser source 14 by sensing the intensity of infrared radiation in the laser chamber is due to background infrared radiation in the laser chamber. That is, an incidental effect of the laser source emitting laser light is that the body of the laser chamber, the air in it and the components in it are heated by the laser light as laser light impinges upon the body of the laser chamber. The heated body of the laser chamber, the heated air in the laser chamber, and the heated components in the chamber emit the background infrared radiation. This background infrared radiation can cause the reading of the optical sensor to be too high, that is, higher than the intensity of the laser light emitted by laser source 14, as the intensity of infrared radiation in the laser chamber sensed by the optical sensor is the sum of the intensity of infrared radiation from the laser light emitted by the laser source and the intensity of the background infrared radiation.
It is an object of the present invention to correct for background infrared radiation in a laser chamber in a closed loop feedback control system for a laser welding system.